Schumann et al. (2003) described ultrastructural defects in heart-stage embryos of the Atpex10 lethal mutant. Extensive defects were apparent in the morphology of the rough ER and in the formation of protein and lipid bodies. Lipid-body membranes, derived from the ER, accumulated in the cytosol. Peroxisomes were not identified in any of the cells. This suggested that At-Pex10p functioned through participation with ER in the formation of all three organelles: lipid bodies, protein bodies, and peroxisomes. Sparkes et al. (2003) also examined the Atpex10 mutant, and came to the more generalized conclusion that AtPex10p was essential for embryo development and viability, but did not specify target organelles or possible involvement of ER. Evidence was obtained, however, for AtPex10p being a peroxin homolog in this and a subsequent paper (Sparkes et al. 2005). Autofluorescent chimeras AtPex10p-YFP and GFP-SKL (SKL is a C-terminal peroxisomal targeting signal), transiently expressed in epidermal cells of tobacco leaves, were co-localized in the tobacco peroxisomes.
A study with endogenous AtPex10p in Arabidopsis cells produced results that were both contrasting and supportive of those of other studies. Flynn et al. (2005) employed immunofluorescence microscopy and semiquantitative immunogold electron microscopy of Arabidopsis cells and sucrose-gradient fractions to elucidate the subcellular location(s) of AtPex10p. As was expected from the predictions of Schumann et al. (2003), but in contrast to negative evidence for trafficking to tobacco-cell ER (Sparkes et al. 2005), AtPex10p was observed in subdomains of ER. Despite numerous varied attempts and approaches, however, they were unable to demonstrate the occurrence of AtPex10p in Arabidopsis peroxisomes. The reason(s) for the apparent discrepancies between these results and those of Sparkes et al. (2003, 2005) are not known.
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